12 DISSEMINATION OF VIRUS 
Insects. Certain insects are carriers of pathogenic bacteria. 
The most important are, perhaps, the mosquito that carries the 
plasmodium of malaria and which is an intermediate host for the 
parasite, and the cattle tick that becomes infected with the piroplasma 
of Texas fever. In some stage, the tick transmits the parasite 
to its offspring which in turn infect susceptible cattle upon which 
they crawl. Many diseases, especially those caused by protozoa, 
are transmitted through the agencies of insects. It is also true 
that certain pathogenic bacteria are carried by flies and other insects. 
Transmission of the virus from the parent to the fetus. Occasionally 
the young of diseased parents are born infected with the disease 
with which one or both of its parents were suffering. In these cases 
the specific organisms were transmitted either from the sire at the 
time of coition, or later to the fetus in the uterus from the dam. 
It is important not to confuse these rare cases with those in which 
the offspring are born uninfected but subsequently contract the 
disease. Many of the so-called hereditary diseases are the result 
of post-natal infection. 
The growing knowledge of infectious diseases is tending to the 
conclusion that the virus of each of them has, through the assistance 
of natural agencies, some one or at most very few, common, so-called 
natural means of gaining entrance to the tissues of a new host. 
While there may be extraneous ways by which infection takes place 
they are, in most instances, relatively unimportant. It is essential, 
therefore, that the more definite manner of dissemination of each 
of the specific diseases of animals be thoroughly understood. This 
involves knowledge of when in the course of the disease the virus 
escapes from the infected animal, what becomes of it after it leaves 
its host and how it gains entrance into the uninfected. 
Diagnosis of a specific infectious disease. The accurate diagnosis 
of a specific infectious disease is made by taking into account one 
or more of the following: 
The symptoms. 
The lesions or morbid anatomy. 
The specific cause. 
Specific reactions. 
Symptoms. ‘The value of symptoms in making a positive diagnosis 
varies with the disease and often with the individual case. While 
each disease exhibits a somewhat constant chain of symptoms it 
